1. Field of the Invention
This invention relates to the measurement of gamma radiation and more specifically to the energy calibration and stabilization of devices for measuring gamma radiation from potassium, uranium, and thorium.
2. Description of the Prior Art
Subsurface gamma ray spectroscopy is an outgrowth of the recording and analysis of natural gamma ray measurements that has occurred since approximately the 1930's. Specifically, subsurface gamma radiation is measured to provide geophysical information of the surrounding subsurface areas. Such information is used in the exploration for petroleum and natural gas.
Historically, three processes have been used to detect and measure gamma radiation. They are photoelectric absorption, Compton scattering and/or pair production. Instruments which embody these processes for measuring gamma radiation include magnetic spectrometers, scintillation spectrometers, proportional gas counters and semiconductors with solid state counters. The present invention uses a CsI (Na) detector in conjunction with a PM tube and appropriate amplifier and digital electronics to produce a pulse height distribution representing the subsurface gamma ray energy spectrum. The scintillation detector and PM tube have been used extensively recently for the measurement of gamma radiation and generates the pulse height spectrum by producing a voltage pulse output whose magnitude is proportional to the energy of the secondary electrons emitted by the gamma ray interaction in the scintillation detector.
Subsurface gamma ray spectroscopy is used to determine the amounts of potassium, uranium and thorium concentrations that naturally occur in geological formations at different subsurface locations. Measurements of gamma radiations from these elements is possible because these elements are associated with radioactive isotopes that emit gamma radiations at characteristics energies. The amount of each element present within a formation can be determined by its contribution to the gamma ray flux at a given energy. Measuring gamma radiation of these specific element concentrations is known as spectral stripping which refers to the subtraction of the contribution of unwanted elements within an energy window, including upper and lower boundaries, set to encompass the characteristic energy(s) of the desired element within the gamma ray energy spectrum. However, measurements of these elements by this method may be complicated by the fact that the energy of a gamma ray photon can be degraded as it passes through matter due to Compton scattering. The consequences are that a photon originally emitted at some given energy within the formation may end up being recorded at a different energy within the measuring device. A further complication is caused by the finite resolution of the gamma ray detection device resulting in a possible smearing of the original photon energy even if the photon energy does reach the detecting device without Compton scattering. Because of these factors, spectral stripping is accomplished in practice by calibrating the tool initially in an artificial formation with known concentrations of potassium, uranium and thorium under standard conditions.
Additionally, energy calibration of the spectrum is continually required while the detection system is traversing subsurface formations because of heat and other environmental factors affecting the measuring device. This continual energy calibration allows the correct placement of energy windows for the purpose of spectral stripping. Traditionally, when the measuring device is underground (or downhole) calibration has been performed by including a known radiating source with the measuring device. However, if the calibrating source emits radiation in the energy range of the potassium, uranium and thorium measured radiation, the calibrating source will corrupt the radiation measurements. If a radiation source is used that emits radiation far away from the energies to be measured, calibration of the measuring device at the potassium, uranium and thorium energy levels is questionable due to multiplied inaccuracies.
The object of the present invention is to maintain the energy calibration of the stripping windows during downhole radiation for measurements of potassium, uranium and thorium concentrations by iteratively recomputing the stripping window boundaries by locating and tracking a naturally occurring spectral peak, such as potassium, at a known energy.